Solar cells can be a viable energy source by utilizing their ability to convert sunlight to electrical energy. Silicon is a semiconductor material used in the manufacture of solar cells; however, a limitation of silicon use relates to the cost of purifying it to solar grade (SG).
Several techniques used to make silicon crystals for solar cells are known. Most of these techniques operate on the principle that while silicon is solidifying from a molten solution, undesirable impurities tend to remain in the molten solution. For example, the float zone technique can be used to make monocrystalline ingots, and uses a moving liquid zone in a solid material, moving impurities to edges of the material. In another example, the Czochralski technique can be used to make monocrystalline ingots, and uses a seed crystal that is slowly pulled out of a solution, allowing the formation of a monocrystalline column of silicon while leaving impurities in the solution. In yet another example, the Bridgeman or heat exchanger techniques can be used to make multicrystalline ingots, and use a temperature gradient to cause directional solidification.
Various techniques for making silicon crystals for solar cells utilize a crucible to hold silicon during the molten manufacturing stage. Unfortunately, most crucibles break after a single use due to, for example, the changing size or shape of the molten silicon as it solidifies. Methods of generating monocrystalline ingots can include the use of a quartz crucible, which is a costly and brittle material. Methods of generating multicrystalline ingots generally use a larger crucible, and due to the expense of quartz, these crucibles are often made of cheaper materials such as fused silica or other refractory materials. Despite being made of cheaper materials, large crucibles made of fused silica or other refractories are still costly to produce, and can generally only be used once. The combination of high expense and limited life of crucibles limits the economic efficiency of silicon purification apparatus and methods.